lib-index: mail-index-strmap - Fix OOB read from truncated record size

[nishat-06]

Truncated record size in mail_index_strmap_read_rec_first over-reads the block

A crafted or corrupted dovecot.index.thread strmap record can pick a packed count where the record byte size count * (sizeof(str_idx) + sizeof(crc32)) overflows the 32-bit rec_size field: with n=0x20000001 the size 0x100000008 truncates to 8, so i_stream_read_bytes(input, ..., rec_size) accepts a tiny block while the code still walks str_idx_base = data + count*sizeof(uint32_t) forward by ~2 GiB and reads count records past it. Widening rec_size to size_t keeps the real byte count, so an oversized record fails the stream read and is rejected as corruption, which is how the sibling doveadm-dump-thread.c path already treats the same quantity.